separate fbdev-specific code out from minui

Isolate the code that interacts with fbdev, in preparation for adding
a new backend.

Change-Id: I19105e9da1ca6408cebc110f7e2bb5abfb481ee9
diff --git a/minui/Android.mk b/minui/Android.mk
index 43e0ad3..5cc84ac 100644
--- a/minui/Android.mk
+++ b/minui/Android.mk
@@ -1,7 +1,7 @@
 LOCAL_PATH := $(call my-dir)
 include $(CLEAR_VARS)
 
-LOCAL_SRC_FILES := graphics.c events.c resources.c
+LOCAL_SRC_FILES := graphics.c graphics_fbdev.c events.c resources.c
 
 LOCAL_C_INCLUDES +=\
     external/libpng\